A device for monitoring doors, which are provided on several floors, of an elevator installation is known from U.S. Pat. No. 6,193,019 B1. In that case, a safety circuit with a chain of switches arranged at the locks of the doors is provided. A resistor is connected in parallel with each of these switches. The respective resistance is bridged over when the switch is closed. The resistance values are formed as a product of a power of two and a reference resistance. The exponent, which is a non-negative integral number, in that case corresponds with the floor on which the door to which the lock with the associated resistance relates is located. If one or more switches are open, then it can thus be calculated from the sum of the resistances at the open switches—which indicate the total resistance with respect thereto of the open safety circuit—which switches are open. This can be output by way of a suitable display device.
The device known from U.S. Pat. No. 6,193,019 B1 for monitoring the doors of an elevator installation has the disadvantage that a significant disturbance, such as a short-circuit, can so impair the functioning of the safety circuit that one or more open locks are wrongly recognized as closed.
A door closure for a door of an elevator is known from EP 1 440 930 A2. The known door closure comprises a housing and a bolt which with respect to the door is transferrable to a locking setting and to a release setting. The bolt is mounted in the housing. The closure comprises a manually actuable emergency unlocking means to transfer the bolt to its release setting, wherein the emergency unlocking means comprises an entrainer which is actuable by an emergency unlocking key to transfer the bolt to the release setting. In addition, provided at or in the housing is a constrainedly opening safety switch or at least a part of a corresponding electrical safety circuit and a manually actuable entrainer, which is associated therewith, for actuation of the safety switch or the safety circuit. In that regard, the door closure can be constructed in such a way that through actuation of the emergency unlocking means initially the safety switch is actuated and subsequently thereto in time thereto the bolt is transferred to its release setting. A normal operation of an item of equipment associated with the door can be interrupted and reinstated by a safety circuit through switching of the safety switch.
Particularly in the case of elevator installations having a reduced shaft head or no shaft head in the elevator shaft, monitoring of door closures, particularly triangle locks, of shaft doors is required. It can thereby be ascertained whether anybody could access the car roof. On opening of a triangle lock, a safety switch is opened, which interrupts a safety circuit. If the safety switch has, as described in EP 1 440 930 A2, a detent function then the elevator installation is stopped until a service engineer places this back in operation after an appropriate investigation. However, if, for example, a short-circuit is now present in the region of the opened switch then the opened setting is not recognized and serious accidents can arise.